Demonstration of putative opiate receptors and their distribution in specific brain regions "in vitro" makes knowledge of the microscopic location of such receptors an essential step in the understanding of the mechanisms of actions of narcotic analgesics. This study will indicate the type, topography and number of cells binding tritiated etorphine "in vivo". It will also demonstrate subcellular location of binding sites. It will indicate if morphologic changes in binding sites can be induced by pretreatment. It will demonstrate any morphologic changes taking place within cells which bind etorphine.